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Unusual isotope effect in the reaction of chlorosilylene with trimethylsilane-1-d: Absolute rate studies and quantum chemical and Rice–Ramsperger–Kassel–Marcus calculations provide strong evidence for the involvement of an intermediate complex

Abstract/Summary

Time-resolved studies of chlorosilylene, ClSiH,
generated by the 193 nm laser flash photolysis of 1-chloro-1-
silacyclopent-3-ene, have been carried out to obtain rate
constants for its bimolecular reaction with trimethylsilane-1-d,
Me3SiD, in the gas phase. The reaction was studied at total
pressures up to 100 Torr (with and without added SF6) over
the temperature range of 295−407 K. The rate constants were
found to be pressure independent and gave the following
Arrhenius equation: log[(k/(cm3 molecule−1 s−1)] = (−13.22
± 0.15) + [(13.20 ± 1.00) kJ mol−1]/(RT ln 10). When
compared with previously published kinetic data for the
reaction of ClSiH with Me3SiH, kinetic isotope effects, kD/kH, in the range from 7.4 (297 K) to 6.4 (407 K) were obtained. These
far exceed values of 0.4−0.5 estimated for a single-step insertion process. Quantum chemical calculations (G3MP2B3 level)
confirm not only the involvement of an intermediate complex, but also the existence of a low-energy internal isomerization
pathway which can scramble the D and H atom labels. By means of Rice−Ramsperger−Kassel−Marcus modeling and a necessary
(but small) refinement of the energy surface, we have shown that this mechanism can reproduce closely the experimental isotope
effects. These findings provide the first experimental evidence for the isomerization pathway and thereby offer the most concrete
evidence to date for the existence of intermediate complexes in the insertion reactions of silylenes.